This invention relates to the dewatering of clays by pressure filtration and more particularly but not exclusively is concerned with the dewatering of aqueous suspensions of white kaolin clays suitable for use as fillers in the manufacture of paper or the like product.
In the manufacture of paper or like product, there is generally incorporated in the fibrous pulp from which the paper or like product is formed a mineral filler the use of which inter alia reduces the cost of the product. One mineral filler which is used for this purpose is white kaolin clay which brings about an improvement in the opacity and printing properties of the paper. However the white kaolin clay fillers conventionally used result in a reduction in the strength of a paper or the like product containing them; and it has recently been proposed that there should be used as the mineral filler for paper or like product a white kaolin clay having a particle size distribution such that the white kaolin clay contains not more than 18% by weight of particles smaller than 2 microns and not more than 15% by weight of particles smaller than 1 micron equivalent spherical diameter. This kaolin clay filler has an unusual particle size distribution in that there are present substantially fewer very small particles as compared with a conventional kaolin clay filler. In the course of preparing such a kaolin clay filler there is formed an aqueous suspension of the kaolin clay filler and it is convenient to complete the preparation of the kaolin clay filler by dewatering the aqueous suspension thereof to reduce the water content of the kaolin clay filler to less than 25% by weight and preferably to less than 20% by weight.
In the dewatering of the aqueous suspension of the kaolin clay filler precautions are taken to prevent as far as possible the formation of fine particles by fracture or shearing of coarser particles. The dewatering is therefore preferably carried out by pressure filtration at a pressure in excess of 150 psig. If a thermal drying step is used subsequent to the pressure filtration step the amount of mechanical work performed on the material in the thermal drying step should be as small as possible and the surface temperature of the material should not be allowed to exceed 120.degree. C. Unfortunately it has been found that in practice it is not possible to eliminate completely from the kaolin clay filler fine particles nor to prevent completely the formation of such fine particles in the dewatering of the aqueous suspension and in other steps in the preparation of the kaolin clay filler.
It is found that when an aqueous suspension of a clay filler (such as the kaolin clay filler described above) which is substantially but not completely free of fine particles is dewatered in a pressure filter the cycle time (i.e. the total time taken for the pressure filter to perform all the operations necessary to form, dewater and discharge a filter cake) increases progressively; and when using a tube pressure filter of the type disclosed in British patent specification No. 1,240,465 the cycle time can increase from about 4 minutes with a new filter cloth to about 15 minutes, at which point the rate of production of filter cake is considered to be unacceptably low. The number of cycles performed in a tube pressure filter before the cycle time rises to this unacceptably high figure varies between about 100 and about 1000 whereas if a tube pressure filter is to be operated economically it is necessary for a filter cloth to remain serviceable for much longer. Similar problems can arise with other types of pressure filter.
It has now been found that the increase in the cycle time is caused by penetration into the inter-filament voids of the filter cloth of fine particles. Although precautions can be taken in the production of a clay filler to keep the amount of fine particles as small as possible, it is not possible to eliminate all fine particles and a few fine particles are inevitably present; and it has been found that these few fine particles are not easily bound to the coarser particles by conventional flocculant (as is the case with a conventional kaolin clay filler have a larger proportion of fine particles) and are therefore free to penetrate the pores of the filter cloth.